Sample preparation conditions for the real-time measurement of W/O emulsions by resonance-enhanced multiphoton ionization time-of-flight mass spectrometry.

Analysis of an emulsion in its original dispersed condition is quite important for quality assessment and quality control. In the present study, the practical experimental conditions of the real-time measurement of a water-in-oil (W/O) emulsion were examined via resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS). A W/O emulsion was prepared using cyclohexane as the oil phase with toluene as an analyte species. A time profile of the peak area for toluene was constructed based on the mass spectra. Normally, the negative spikes of a base signal are detected in a time profile when analyte molecules are dispersed in an oil phase. In this case, however, the positive spikes were unexpectedly detected rather than the negative ones. Though several factors could be relevant for the occurrence of the positive spikes, these spikes could have been suppressed by the addition of a small amount of n-alkane when the oil phase was prepared in the present study. The practical experimental conditions for the analysis of a W/O emulsion in real-time revealed that this method would be applicable to the analysis of an oil-in-water-in-oil (O/W/O) emulsion where the outer phase is also an oil phase.

Using Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry to Evaluate the Movement of a Constituent in a Multiple Emulsion.

Herein, we propose a method for evaluating the movement of a constituent in a multiple emulsion while maintaining its original dispersed condition. In this study, an oil-in-water-in-oil (O1/W/O2) emulsion was prepared using a two-step emulsification method with styrene as an analyte species in the inner phase (O1). The emulsion was measured using resonance-enhanced multiphoton ionization time-of-flight mass spectrometry without pretreatment such as centrifugation. From a series of obtained mass spectra, a time profile for the peak areas arising from styrene was constructed. When the emulsion was measured immediately following preparation, a time profile composed of a base, positive, and negative signals confirmed the presence of styrene in the O2, O1, and W phases, respectively. Moreover, while a small amount of styrene was present in the inner O1 phase, almost all of the styrene was found in the outer O2 phase. Furthermore, the results of the obtained time profile were converted into a box plot, and a method for the selection of the base, positive, and negative signals was tentatively determined. Then, the movement of styrene among the phases could be evaluated using the time courses of these signals; the time constant of the movement of styrene from an O1/W droplet to the O2 phase was calculated to be 0.8 h.